An activated sludge system maintains a total solids inventory of 24,000 lbs MLSS. If waste activated sludge (WAS) removes 2,800 lbs/day and secondary effluent loses 200 lbs/day, what is the Mean Cell Residence Time (MCRT) in days?

Formula Used: MCRT (days) = [Total System MLSS (lbs)] ÷ [WAS Solids (lbs/day) + Effluent Solids (lbs/day)]

Step-by-Step Mathematical Derivation:

1. Calculate total daily solids leaving the system = 2,800 lbs/day (wasted) + 200 lbs/day (effluent loss) = 3,000 lbs/day.
2. MCRT = 24,000 lbs ÷ 3,000 lbs/day = 8.0 days.

⚠️ Common Exam Trap: Always include BOTH intentionally wasted WAS solids AND unintentional effluent suspended solids in the denominator.